Devices for converting a three phase power input to an adjustable direct current output generally include a rectifier stage for converting the three phase alternating current (AC) input into a direct current (DC) output, and a DC-DC conversion stage for adjusting the direct current output. The DC-DC conversion stage may be capable of raising or lowering the DC voltage level, or both, depending on the particular features of a given device.
In many applications, and particularly high power applications, it is desirable that power conversion circuitry provide power factor correction to ensure high power efficiency and to minimize the input current. Power factor correction prevents harmonic currents from distorting the supplied power waveform, thereby keeping both input voltage and current waveforms in phase and maintaining the apparent power of the three phase power input. Power efficiency becomes increasingly important for high power applications, since more power that is demanded at a given percentage of efficiency, the more power is lost in the conversion process. Lost or dissipated power is not only wasteful, but can also introduce unwanted heat to the power conversion circuitry. Cooling equipment could then be required to cool the circuitry, which may in turn add cost, increase weight and take up valuable space. The weight and space of the power conversion circuitry may be especially important in aircraft or spacecraft applications, in which conservation of weight and/or space can in turn improve the flight of and reduce the fuel requirements of an aircraft or spacecraft on which the circuitry is installed.
It is yet further desirable that the power conversion circuitry poses a low technical risk, thereby reducing the risk of failure, and also reducing the cost of designing, manufacturing and maintaining the circuitry.
It is also desirable that the circuitry be operable in applications for which high frequency power is supplied, such as the 115V 400 Hz AC power commonly used for aircraft.
FIG. 1 is a functional block diagram representation of one power conversion device known in the art for converting a three-phase AC input to a DC output while achieving power factor correction with a power efficiency of about 90%. The device of FIG. 1 includes three separate full bridge rectifiers 112-116, each rectifier taking in a different phase input AC1, AC2 and AC3 of the three phase AC source. The rectified output of each rectifier 112-116 is then fed to a respective active power factor correction (APFC) converter 122-126, which provides power factor correction for the rectified current and voltage. Each APFC converter 122-126 is connected to a respective DC-DC converter 132-136, which receives the output of the converter, adjusts the voltage level of the output, and electrically isolates (i.e., floats) the adjusted voltage from the converter. Load sharing of the respective floated outputs of the DC-DC converters are managed by controllers 142-146, and combined into a single DC output. Thus, the device of FIG. 1 achieves power factor correction, electrical isolation, and power regulation (with a second conversion).
However, the device of FIG. 1 requires nine independent control circuits (one for each APFC, one for each DC-DC converter, and one for each load sharing controller) to operate. This adds unwanted cost, weight and space to the design. The device also requires a lot of bus capacitance, and requires not one but two stages of DC-DC conversion to yield the converted power. Moreover, the control circuitry is complex and, therefore, poses a high cost and a high technical risk.
FIG. 2 is a functional block diagram representation of a Vienna rectifier, another power conversion device known in the art for converting a three-phase AC input to a DC output while achieving power factor correction with an improved power efficiency (relative to the device of FIG. 1. The device of FIG. 2 includes three switch controlled rectifier circuits 212-216 and a control circuit 220. The control circuit receives as inputs the input three phase AC power and the output DC power. Based on a complex calculation, the control circuit 220 uses these inputs to determine a separate control instruction for each switch of the respective rectifier circuits 212-216. The rectified outputs of the rectifier circuits 212-216 are combined to yield the DC output of the device. Thus, the device of FIG. 2 achieves power factor correction and power regulation (although the output voltage may only be stepped up and not down) with a single control circuit and three switches.
However, the device of FIG. 2 does not achieve electrical isolation, and still requires a complex control circuit, which adds unwanted cost, technical risk, and some weight and space to the design.
Accordingly, there is a need for a smaller, lighter, less expensive, and less complex power conversion circuit that achieves power factor correction, power regulation and electrical isolation, preferably with at least the same or better power efficiency.